TypeError: can only concatenate str(not "NoneType") to str error using stack to reverse string

• reverse=reverse+stack.pop() appears to have the following error:

  TypeError: can only concatenate str(not "NoneType") to str
  

• Why do you use stack.size() in while stack.size()?
  Does this mean that the character "Hello" is repeated for a few minutes (5 times)?
  Also, do you mean stack.size() until the contents are gone?

TypeError: can only concatenate str(not "NoneType") to str

 class Stack:
    def__init__(self):
        self.items = [ ]
    def push (self, item):
        self.items.append(item)
    def pop(self):
        self.items.pop()
    def size (self):
        return len(self.items)

stack = Stack()

for c in "Hello":
    stack.push(c)

reverse=""

while stack.size():
    reverse=reverse+stack.pop()

print(reverse)